Method for reducing metal content of self-supporting composite bodies and articles formed thereby

ABSTRACT

This invention relates generally to a novel method for removing metal from a formed self-supporting body. A self-supporting body is made by reactively infiltrating a molten parent metal into a bed or mass containing a boron source material and a carbon source material (e.g., boron carbide) and/or a boron source material and a nitrogen source material (e.g., boron nitride) and, optionally, one or more inert fillers. Once the self-supporting body is formed, it is then placed, at least partially, into contact with another material which causes metallic constituent contained in the self-supporting body to be at least partially removed.

This application is a continuation of copending application Ser. No.07/551,288 filed on Jul. 12, 1990, now abandoned.

FIELD OF THE INVENTION

This invention relates generally to a novel method for removing metalfrom a formed self-supporting body. A self-supporting body is made byreactively infiltrating a molten parent metal into a bed or masscontaining a boron source material and a carbon source material (e.g.,boron carbide) and/or a boron source material and a nitrogen sourcematerial (e.g., boron nitride) and, optionally, one or more inertfillers. Once the self-supporting body is formed, it is then placed, atleast partially, into contact with another material which causesmetallic constituent contained in the self-supporting body to be atleast partially removed.

BACKGROUND OF THE PRESENT INVENTION

In recent years, there has been an increasing interest in the use ofceramics for structural applications historically served by metals. Theimpetus for this interest has been the superiority of ceramics withrespect to certain properties, such as corrosion resistance, hardness,wear resistance, modulus of elasticity, and refractory capabilities whencompared with metals.

However, a major limitation on the use of ceramics for such purposes isthe feasibility and cost of producing the desired ceramic structures.For example, the production of ceramic boride bodies by the methods ofhot pressing, reaction sintering and reaction hot pressing is wellknown. In the case of hot pressing, fine powder particles of the desiredboride are compacted at high temperatures and pressures. Reaction hotpressing involves, for example, compacting at elevated temperatures andpressures boron or a metal boride with a suitable metal-containingpowder. U.S. Pat. No. 3,937,619 to Clougherty describes the preparationof a boride body by hot pressing a mixture of powdered metal with apowdered diboride, and U.S. Pat. No. 4,512,946 to Brun describes hotpressing ceramic powder with boron and a metal hydride to form a boridecomposite.

However, these hot pressing methods require special handling andexpensive special equipment, they are limited as to the size and shapeof the ceramic part produced, and they typically involve low processproductivities and high manufacturing cost.

A second major limitation on the use of ceramics for structuralapplications is their general lack of toughness (i.e. damage toleranceor resistance to fracture). This characteristic tends to result insudden, easily induced, catastrophic failure of ceramics in applicationsinvolving even rather moderate tensile stresses. This lack of toughnesstends to be particularly common in monolithic ceramic boride bodies.

One approach to overcome this problem has been to attempt to useceramics in combination with metals, for example, as cermets or metalmatrix composites. The objective of this approach is to obtain acombination of the best properties of the ceramic (e.g. hardness and/orstiffness) and the metal (e.g. ductility). U.S. Pat. No. 4,585,618 toFresnel, et al., discloses a method of producing a cermet whereby a bulkreaction mixture of particulate reactants, which react to produce asintered self-sustaining ceramic body, is reacted while in contact witha molten metal. The molten metal infiltrates at least a portion of theresulting ceramic body. Exemplary of such a reaction mixture is onecontaining titanium, aluminum and boron oxide (all in particulate form),which is heated while in contact with a pool of molten aluminum. Thereaction mixture reacts to form titanium diboride and alumina as theceramic phase, which is infiltrated by the molten aluminum. Thus, thismethod uses the aluminum in the reaction mixture principally as areducing agent. Further, the external pool of molten aluminum is notbeing used as a source of precursor metal for a boride forming reaction,but rather it is being utilized as a means to fill the pores in theresulting ceramic structure. This creates cermets which are wettable andresistant to molten aluminum. These cermets are particularly useful inaluminum production cells as components which contact the moltenaluminum produced but preferably remain out of contact with the moltencryolite.

European Application 0,113,249 to Reeve, et al. discloses a method formaking a cermet by first forming in situ dispersed particles of aceramic phase in a molten metal phase, and then maintaining this moltencondition for a time sufficient to effect formation of an intergrownceramic network. Formation of the ceramic phase is illustrated byreacting a titanium salt with a boron salt in a molten metal such asaluminum. A ceramic boride is developed in situ and becomes anintergrown network. There is, however, no infiltration, and further theboride is formed as a precipitate in the molten metal. Both examples inthe application expressly state that no grains were formed of TiAl₃,AlB₂, or AlB₁₂, but rather TiB₂ is formed demonstrating the fact thatthe aluminum is not the metal precursor to the boride.

U.S. Pat. No. 3,864,154 to Gazza, et al. discloses a ceramic-metalsystem produced by infiltration. An AlB₁₂ compact was impregnated withmolten aluminum under vacuum to yield a system of these components.Other materials prepared included SiB₆ -Al, B-Al; B₄ C-Al/Si; and AlB₁₂-B-Al. There is no suggestion whatsoever of a reaction, and nosuggestion of making composites involving a reaction with theinfiltrating metal nor of any reaction product embedding an inert filleror being part of a composite.

U.S. Pat. No. 4,605,440 to Halverson, et al., discloses that in order toobtain B₄ C-Al composites, a B₄ C-Al compact (formed by cold pressing ahomogeneous mixture of B₄ C and Al powders) is subjected to sintering ineither a vacuum or an argon atmosphere. There is no infiltration ofmolten metal from a pool or body of molten precursor metal into apreform. Further, there is no mention of a reaction product embedding aninert filler in order to obtain composites utilizing the favorableproperties of the filler.

While these concepts for producing cermet materials have in some casesproduced promising results, there is a general need for more effectiveand economical methods to prepare boride-containing materials.

DISCUSSION OF RELATED PATENTS AND PATENT APPLICATIONS

Many of the above-discussed problems associated with the production ofboride-containing materials have been addressed in U.S. Pat. No.4,885,130 (hereinafter "Patent '130"), which issued on Dec. 5, 1989, inthe names of Danny R. White, Michael K. Aghajanian and T. Dennis Claar,and is entitled "Process for Preparing Self-Supporting Bodies andProducts Made Thereby".

Briefly summarizing the disclosure of Patent '130, self-supportingceramic bodies are produced by utilizing a parent metal infiltration andreaction process (i.e., reactive infiltration) in the presence of a masscomprising boron carbide. Particularly, a bed or mass comprising boroncarbide and, optionally, one or more of a boron donor material and acarbon donor material, is infiltrated by molten parent metal, and thebed may be comprised entirely of boron carbide or only partially ofboron carbide, thus resulting in a self-supporting body comprising, atleast in part, one or more parent metal boron-containing compounds,which compounds include a parent metal boride or a parent metal borocarbide, or both, and typically also may include a parent metal carbide.It is also disclosed that the mass comprising boron carbide which is tobe infiltrated may also contain one or more inert fillers mixed with theboron carbide. Accordingly, by combining an inert filler, the resultwill be a composite body having a matrix produced by the reactiveinfiltration of the parent metal, said matrix comprising at least oneboron-containing compound, and the matrix may also include a parentmetal carbide, the matrix embedding the inert filler. It is furthernoted that the final composite body product in either of theabove-discussed embodiments (i.e., filler or no filler) may include aresidual metal as at least one metallic constituent of the originalparent metal.

Broadly, in the disclosed method of Patent '130, a mass comprising boroncarbide and, optionally, one or more of a boron donor material and acarbon donor material, is placed adjacent to or in contact with a bodyof molten metal or metal alloy, which is melted in a substantially inertenvironment within a particular temperature envelope. The molten metalinfiltrates the mass comprising boron carbide and reacts with at leastthe boron carbide to form at least one reaction product. The boroncarbide (and/or the boron donor material and/or the carbon donormaterial) is reducible, at least in part, by the molten parent metal,thereby forming the parent metal boron-containing compound (e.g., aparent metal boride and/or boro compound under the temperatureconditions of the process). Typically, a parent metal carbide is alsoproduced, and in certain cases, a parent metal boro carbide is produced.At least a portion of the reaction product is maintained in contact withthe metal, and molten metal is drawn or transported toward the unreactedmass comprising boron carbide by a wicking or a capillary action. Thistransported metal forms additional parent metal, boride, carbide, and/orboro carbide and the formation or development of a ceramic body iscontinued until either the parent metal or mass comprising boron carbidehas been consumed, or until the reaction temperature is altered to beoutside of the reaction temperature envelope. The resulting structurecomprises one or more of a parent metal boride, a parent metal borocompound, a parent metal carbide, a metal (which, as discussed in Patent'130, is intended to include alloys and intermetallics), or voids, orany combination thereof. Moreover, these several phases may or may notbe interconnected in one or more dimensions throughout the body. Thefinal volume fractions of the boron-containing compounds (i.e., borideand boron compounds), carbon-containing compounds, and metallic phases,and the degree of interconnectivity, can be controlled by changing oneor more conditions, such as the initial density of the mass comprisingboron carbide, the relative amounts of boron carbide and parent metal,alloys of the parent metal, dilution of the boron carbide with a filler,the amount of boron donor material and/or carbon donor material mixedwith the mass comprising boron carbide, temperature, and time.Preferably, conversion of the boron carbide to the parent metal boride,parent metal boro compound(s) and parent metal carbide is at least about50%, and most preferably at least about 90%.

The typical environment or atmosphere which was utilized in Patent '130was one which is relatively inert or unreactive under the processconditions. Particularly, it was disclosed that an argon gas, or avacuum, for example, would be suitable process atmospheres. Stillfurther, it was disclosed that when zirconium was used as the parentmetal, the resulting composite comprised zirconium diboride, zirconiumcarbide, and residual zirconium metal. It was also disclosed that whenaluminum parent metal was used with the process, the result was analuminum boro carbide such as Al₃ B₄₈ C₂, AlB₁₂ C₂ and/or AlB₂₄ C₄, withaluminum parent metal and other unreacted unoxidized constituents of theparent metal remaining. Other parent metals which were disclosed asbeing suitable for use with the processing conditions included silicon,titanium, hafnium, lanthanum, iron, calcium, vanadium, niobium,magnesium, and beryllium.

Still further, it is disclosed that by adding a carbon donor material(e.g., graphite powder or carbon black) and/or a boron donor material(e.g., a boron powder, silicon borides, nickel borides and iron borides)to the mass comprising boron carbide, the ratio of parentmetal-boride/parent metal-carbide can be adjusted. For example, ifzirconium is used as the parent metal, the ratio of ZrB₂ /ZrC can bereduced if a carbon donor material is utilized (i.e., more ZrC isproduced due to the addition of a carbon donor material in the mass ofboron carbide) while if a boron donor material is utilized, the ratio ofZrB₂ /ZrC can be increased (i.e., more ZrB₂ is produced due to theaddition of a boron donor material in the mass of boron carbide). Stillfurther, the relative size of ZrB₂ platelets which are formed in thebody may be larger than platelets that are formed by a similar processwithout the use of a boron donor material. Thus, the addition of acarbon donor material and/or a boron donor material may also effect themorphology of the resultant material.

In another related Patent, specifically, U.S. Pat. No. 4,915,736(hereinafter referred to as "Patent '736"), issued in the names of TerryDennis Claar and Gerhard Hans Schiroky, on Apr. 10, 1990, and entitled"A Method of Modifying Ceramic Composite Bodies By a CarburizationProcess and Articles Made Thereby", additional modification techniquesare disclosed. Specifically, Patent '736 discloses that a ceramiccomposite body made in accordance with the teachings of, for example,Patent '130 can be modified by exposing the composite to a gaseouscarburizing species. Such a gaseous carburizing species can be producedby, for example, embedding the composite body in a graphitic bedding andreacting at least a portion of the graphitic bedding with moisture oroxygen in a controlled atmosphere furnace. However, the furnaceatmosphere should comprise typically, primarily, a non-reactive gas suchas argon. It is not clear whether impurities present in the argon gassupply the necessary O₂ forming a carburizing species, or whether theargon gas merely O₂ for serves as a vehicle which contains impuritiesgenerated by some type of volatilization of components in the graphiticbedding or in the composite body. In addition, a gaseous carburizingspecies could be introduced directly into a controlled atmospherefurnace during heating of the composite body.

Once the gaseous carburizing species has been introduced into thecontrolled atmosphere furnace, the setup should be designed in such amanner to permit the carburizing species to be able to contact at leasta portion of the surface of the composite body buried in the looselypacked graphitic powder. It is believed that carbon in the carburizingspecies, or carbon from the graphitic bedding, will dissolve into theinterconnected zirconium carbide phase, which can then transport thedissolved carbon throughout substantially all of the composite body, ifdesired, by a vacancy diffusion process. Moreover, Patent '736 disclosesthat by controlling the time, the exposure of the composite body to thecarburizing species and/or the temperature at which the carburizationprocess occurs, a carburized zone or layer can be formed on the surfaceof the composite body. Such process could result in a hard,wear-resistant surface surrounding a core of composite material having ahigher metal content and higher fracture toughness.

Thus, if a composite body was formed having a residual parent metalphase in the amount of between about 5-30 volume percent, such compositebody could be modified by a post-carburization treatment to result infrom about 0 to about 2 volume percent, typically about 1/2 to about 2volume percent, of parent metal remaining in the composite body.

U.S. Pat. No. 4,885,131 (hereinafter "Patent '131"), issued in the nameof Marc S. Newkirk on Dec. 5, 1989, and entitled "Process For PreparingSelf-Supporting Bodies and Products Produced Thereby", disclosesadditional reactive infiltration formation techniques. Specifically,Patent '131 discloses that self-supporting bodies can be produced by areactive infiltration of a parent metal into a mixture of a bed or masscomprising a boron donor material and a carbon donor material. Therelative amounts of reactants and process conditions may be altered orcontrolled to yield a body containing varying volume percents ofceramic, metals, ratios of one ceramic or another and porosity.

In another related patent application, specifically, copending U.S.patent application Ser. No. 07/296,770 (hereinafter referred to as"Application '770"), filed in the names of Terry Dennis Claar et al., onJan. 13, 1989, and entitled "A Method of Producing Ceramic CompositeBodies", additional reactive infiltration formation techniques aredisclosed. Specifically, Application '770 discloses various techniquesfor shaping a bed or mass comprising boron carbide into a predeterminedshape and thereafter reactively infiltrating the bed or mass comprisingboron carbide to form a self-supporting body of a desired size andshape.

Copending U.S. patent application Ser. No. 07/296,837 (hereinafterreferred to as "Application '837"), filed in the name of Terry DennisClaar on Jan. 13, 1989, and entitled "A Method of Bonding A CeramicComposite Body to a Second Body and Articles Produced Thereby",discloses various bonding techniques for bonding self-supporting bodiesto second materials. Particularly, this patent application disclosesthat a bed or mass comprising one or more boron-containing compounds isreactively infiltrated by a molten parent metal to produce aself-supporting body. Moreover, residual or excess metal is permitted toremain bonded to the formed self-supporting body. The excess metal isutilized to form a bond between the formed self-supporting body andanother body (e.g., a metal body or a ceramic body of any particularsize or shape).

The reactive infiltration of a parent metal into a bed or masscomprising boron nitride is disclosed in copending U.S. Pat. No.4,904,446 (hereinafter "Patent '446"), issued in the names of Danny RayWhite et al., on Feb. 27, 1990, and entitled "Process for PreparingSelf-Supporting Bodies and Products Made Thereby". Specifically, thispatent discloses that a bed or mass comprising boron nitride can bereactively infiltrated by a parent metal. A relative amount of reactantsand process conditions may be altered or controlled to yield a bodycontaining varying volume percents of ceramic, metal and/or porosity.Additionally, the self-supporting body which results comprises aboron-containing compound, a nitrogen-containing compound and,optionally, a metal. Additionally, inert fillers may be included in theformed self-supporting body.

A further post-treatment process for modifying the properties ofproduced ceramic composite bodies is disclosed in copending U.S. patentapplication Ser. No. 07/296,966 (hereinafter "Application '966"), filedin the names of Terry Dennis Claar et al., on Jan. 13, 1989, andentitled "A Method of Modifying Ceramic Composite Bodies ByPost-Treatment Process and Articles Produced Thereby". Specifically,Application '966 discloses that self-supporting bodies produced by areactive infiltration technique can be post-treated by exposing theformed bodies to one or more metals and heating the exposed bodies tomodify at least one property of the previously formed composite body.One specific example of a post-treatment modification step includesexposing a formed body to a siliconizing environment.

Copending U.S. patent application Ser. No. 07/296,961 (hereinafter"Application '961"), filed in the names of Terry Dennis Claar et al., onJan. 13, 1989, and entitled "A Process for Preparing Self-SupportingBodies Having Controlled Porosity and Graded Properties and ProductsProduced Thereby", discloses reacting a mixture of powdered parent metalwith a bed or mass comprising boron carbide and, optionally, one or moreinert fillers. Additionally, it is disclosed that both a powdered parentmetal and a body or pool of molten parent metal can be induced to reactwith a bed or mass comprising boron carbide. The body which is producedis a body which has controlled or graded properties.

The disclosures of each of the above-discussed Commonly Owned U.S.Patent Applications and Patents are herein expressly incorporated byreference.

SUMMARY OF THE INVENTION

In accordance with a first step of the present invention,self-supporting ceramic bodies are produced by utilizing a parent metalinfiltration and reaction process (i.e. reactive infiltration) in thepresence of a bed or mass comprising, for example, boron carbide orboron nitride. Such bed or mass is infiltrated by molten parent metal,and the bed may be comprised entirely of boron carbide, boron nitride,and/or mixtures of boron donor materials and carbon donor materials.Depending on the particular reactants involved in the reactiveinfiltration, the resulting bodies which are produced comprise one ormore reaction products of parent metal boron-containing compounds,and/or one or more parent metal carbon-containing compounds and/or oneor more parent metal nitrogen-containing compounds, etc. Alternatively,the mass to be infiltrated may contain one or more inert fillers admixedtherewith to produce a composite by reactive infiltration, whichcomposite comprises a matrix of one or more of the aforementionedreaction products and also may include residual unreacted or unoxidizedconstituents of the parent metal. The filler material may be embedded bythe formed matrix. The final product may include a metal as one or moremetallic constituents of the parent metal. Still further, in some casesit may be desirable to add a carbon donor material (i.e., acarbon-containing compound) and/or a boron donor material (i.e., aboron-containing compound) to the bed or mass which is to be infiltratedto modify, for example, the relative amounts of one formed reactionproduct to another, thereby modifying resultant mechanical properties ofthe composite body. Still further, the reactant concentrations andprocess conditions may be altered or controlled to yield a bodycontaining varying volume percents of ceramic compounds, metal and/orporosity.

Broadly, in accordance with the first step of the method according tothis invention, the bed or mass which is to be reactively infiltratedmay be placed adjacent to or contacted with a body of molten metal ormetal alloy, which is melted in a substantially inert environment withina particular temperature envelope. The molten metal infiltrates the massand reacts with at least one constituent of the bed or mass to beinfiltrated to form one or more reaction products. At least a portion ofthe formed reaction product is maintained in contact with the metal, andmolten metal is drawn or transported toward the remaining unreacted massby a wicking or capillary action. This transported metal formsadditional reaction product upon contact with the remaining unreactedmass, and the formation or development of a ceramic body is continueduntil the parent metal or remaining unreacted mass has been consumed, oruntil the reaction temperature is altered to be outside the reactiontemperature envelope. The resulting structure comprises, depending uponthe particular materials comprising the bed or mass which is to bereactively infiltrated, one or more of a parent metal boride, a parentmetal boro compound, a parent metal carbide, a parent metal nitride, ametal (which as used herein is intended to include alloys andintermetallics), or voids, or a combination thereof, and these severalphases may or may not be interconnected in one or more dimensions. Thefinal volume fractions of the reaction products and metallic phases, andthe degree of interconnectivity, can be controlled by changing one ormore conditions, such as the initial density of the mass to bereactively infiltrated, the relative amounts and chemical composition ofthe materials contained within the mass which is to be reactivelyinfiltrated, the amount of parent metal provided for reaction, thecomposition of the parent metal, the presence and amount of one or morefiller materials, temperature, time, etc.

Typically, the mass to be reactively infiltrated should be at leastsomewhat porous so as to allow for wicking the parent metal through thereaction product. Wicking occurs apparently either because any volumechange on reaction does not fully close off pores through which parentmetal can continue to wick, or because the reaction product remainspermeable to the molten metal due to such factors as surface energyconsiderations which render at least some of its grain boundariespermeable to the parent metal.

In another aspect of the first step of the invention, a composite isproduced by the transport of molten parent metal into the bed or masswhich is to be reactively infiltrated which has admixed therewith one ormore inert filler materials. In this embodiment, one or more suitablefiller materials are mixed with the bed or mass to be reactivelyinfiltrated. The resulting self-supporting ceramic-metal composite thatis produced typically comprises a dense microstructure which comprises afiller embedded in a matrix comprising at least one parent metalreaction product, and also may include a substantial quantity of metal.Typically, only a small amount of material (e.g., a small mount of boroncarbide) is required to promote the reactive infiltration process. Thus,the resulting matrix can vary in content from one composed primarily ofmetallic constituents thereby exhibiting certain propertiescharacteristic of the parent metal; to cases where a high concentrationof reaction product is formed, which dominates the properties of thematrix. The filler may serve to enhance the properties of the composite,lower the raw materials cost of the composite, or moderate the kineticsof the reaction product formation reactions and the associated rate ofheat evolution. The precise starting amounts and composition ofmaterials utilized in the reaction infiltration process can be selectedso as to result in a desirable body which is compatible with the secondstep of the invention.

In another aspect of the first step of the present invention, thematerial to be reactively infiltrated is shaped into a preformcorresponding to the geometry of the desired final composite. Subsequentreactive infiltration of the preform by the molten parent metal resultsin a composite having the net shape or near net shape of the preform,thereby minimizing expensive final machining and finishing operations.Moreover, to assist in reducing the amount of final machining andfinishing operations, a barrier material can at least partially, orsubstantially completely, surround the preform. The use of a graphitematerial (e.g., a graphite mold, a graphite tape product, a graphitecoating, etc.) is particularly useful as a barrier for such parentmetals as zirconium, titanium, or hafnium, when used in combination withpreforms made of, for example, boron carbide, boron nitride, boron andcarbon. Still further, by placing an appropriate number of through-holeshaving a particular size and shape in the aforementioned graphite mold,the amount of porosity which typically occurs within a composite bodymanufactured according to the first step of the present invention, canbe reduced. Typically, a plurality of holes is placed in a bottomportion of the mold, or that portion of the mold toward which reactiveinfiltration occurs. The holes function as a venting means which permitthe removal of, for example, argon gas which has been trapped in thepreform as the parent metal reactive infiltration front infiltrates thepreform.

Still further, the procedures discussed above herein in the Section"Discussion of Related Patents and Patent Applications" may beapplicable in connection with the first step of the present invention.

Once a self-supporting body has been formed in accordance with the firststep of the present invention, then the second step of the presentinvention is put into effect. The second step of the present inventioninvolves contacting at least a portion of the formed self-supportingbody with a material which causes at least a portion of the metallicconstituent to be at least partially removed from the self-supportingbody.

In a first embodiment of the invention, a metallic constituent of aself-supporting composite body produced in accordance with the firststep of the present invention can be at least partially, orsubstantially completely, removed by causing the metallic constituent toreact with an adjacent permeable mass of material. To achieve removal ofthe metallic constituent, at least a portion of the permeable mass isplaced into contact with at least a portion of the metallic constituentcontained within the self-supporting body. Thus, at least a portion ofthe metallic constituent should be at least partially accessible, orshould be made to be at least partially accessible, from at least onesurface of the self-supporting composite body.

The amount or selected portion of metallic constituent which is causedto be removed from the self-supporting body can be controlled to achievea desirable metal content. Specifically, substantially all metallicconstituent located in a certain area within a self-supporting compositebody (e.g., located near a surface of the self-supporting compositebody) may be substantially completely removed from that selected area,thereby leaving other areas of metallic constituent within the compositebody substantially undisturbed. Moreover, if the metallic constituent issubstantially interconnected throughout the composite body,substantially all the metallic constituent could be removed. Thevolumetric amount of metallic constituent to be removed from theself-supporting composite body depends upon the ultimate application forthe composite body. Thus, the present invention may be utilized merelyas a surface modification process, or it could be used to removesubstantially all the metallic constituent from a self-supportingcomposite body.

In a preferred embodiment, the second step of the present invention, theself-supporting body may be substantially completely surrounded by andcontacted with an appropriate material. In this embodiment, at least aportion of, or substantially all of, the metallic constituent could beremoved from substantially all surfaces of the self-supporting body solong as the metallic constituent is at least partially accessible, orcan be made to be at least partially accessible from such surfaces.

In another preferred embodiment of the second step of the presentinvention, only a portion of the self-supporting body may be contactedwith the appropriate mass of material. In this preferred embodiment, themetallic constituent could be selectively remove from that surface whichis in contact with the permeable mass. In this preferred embodiment, itis possible to achieve a grading of properties within a self-supportingbody from one side of the body relative to another side of the body.Such grading could permit the self-supporting body to be used for anumber of different applications.

A number of materials may be placed into contact with self-supportingbodies formed in accordance with the first step of the presentinvention. Acceptable materials include carbide, nitrides, borides, etc.A primary selection criteria for the material comprising the permeablemass is that the permeable mass should be wetable by the metalliccomponent of the self-supporting body. Moreover, the permeable mass canbe selected so that it is substantially nonreactive with or veryreactive with the metallic component of the self-supporting body. In thecase where the permeable mass is selected so that it is substantiallynonreactive with the metallic constituent comprising a self-supportingbody, very little conversion of metallic constituent to another phasecan be expected; whereas if a metallic constituent is reactive with amaterial in the permeable mass, partial conversion of the metallicconstituent to another material can be expected.

As stated above, the amount of metallic constituent that is removed froma self-supporting body can be controlled to be within any particulardesirable range. For example, if a self-supporting body was formed tocontain about 20 volume percent metallic constituent, substantially allof the metallic constituent could be removed by following the teachingsof the present invention. Additionally, it has been observed, that whenthe material comprising the permeable mass is substantially nonreactive(e.g., chemically) with metallic constituent contained in theself-supporting body, that substantially no conversion of metallicconstituent to another material occurs. But rather, substantiallycomplete removal of the metallic constituent from the self-supportingbody is essentially all that occurs. This fact has been proven byquantitative image analysis.

DEFINITIONS

As used in this specification and the appended claims, the terms beloware defined as follows:

"Parent metal" refers to that metal, e.g. zirconium, which is theprecursor for the polycrystalline oxidation reaction product, that is,the parent metal boride, parent metal nitride, or other parent metalcompound, and includes that metal as a pure or relatively pure metal, acommercially available metal having impurities and/or alloyingconstituents therein, and an alloy in which that metal precursor is themajor constituent; and when a specific metal is mentioned as the parentmetal, e.g. zirconium, the metal identified should be read with thisdefinition in mind unless indicated otherwise, by the context.

"Parent metal boride" and "parent metal boro compounds" mean a reactionproduct containing boron formed upon reaction between boron nitride andthe parent metal and includes a binary compound of boron with the parentmetal as well as ternary or higher order compounds.

"Parent metal nitride" means a reaction product containing nitrogenformed upon reaction of boron nitride and parent metal.

"Parent metal carbide" means a reaction product containing carbon formedupon reaction of a carbon source and parent metal.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic view of the setup used to fabricate a plateletreinforced composite body;

FIG. 2 is a schematic view of the setup used to remove the residualmetal from the platelet reinforced composite body; and

FIG. 3 is an approximately 100× magnification photomicrograph of across-section of the platelet reinforced composite body after the metalremove process.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

In accordance with a first step of the present invention,self-supporting ceramic bodies are produced by utilizing a parent metalinfiltration and reaction process (i.e. reactive infiltration) in thepresence of a bed or mass comprising, for example, boron carbide orboron nitride. Such bed or mass is infiltrated by molten parent metal,and the bed may be comprised entirely of boron carbide, boron nitride,and/or mixtures of boron donor materials and carbon donor materials.Depending on the particular reactants involved in the reactiveinfiltration, the resulting bodies which are produced comprise one ormore reaction products of parent metal boron-containing compounds,and/or one or more parent metal carbon-containing compounds and/or oneor more parent metal nitrogen-containing compounds, etc. Alternatively,the mass to be infiltrated may contain one or more inert fillers admixedtherewith to produce a composite by reactive infiltration, whichcomposite comprises a matrix of one or more of the aforementionedreaction products and also may include residual unreacted or unoxidizedconstituents of the parent metal. The filler material may be embedded bythe formed matrix. The final product may include a metal such as one ormore metallic constituents of the parent metal. Still further, in somecases it may be desirable to add a carbon donor material (i.e., acarbon-containing compound) and/or a boron donor material (i.e., aboron-containing compound) to the bed or mass which is to be infiltratedto modify, for example, the relative amounts of one formed reactionproduct to another, thereby modifying resultant mechanical properties ofthe composite body. Still further, the reactant concentrations andprocess conditions may be altered or controlled to yield a bodycontaining varying volume percents of ceramic compounds, metal and/orporosity.

Broadly, in accordance with the first step of the method according tothis invention, the bed or mass which is to be reactively infiltratedmay be placed adjacent to or contacted with a body of molten metal ormetal alloy, which is melted in a substantially inert environment withina particular temperature envelope. The molten metal infiltrates the massand reacts with at least one constituent of the bed or mass to beinfiltrated to form one or more reaction products. At least a portion ofthe formed reaction product is maintained in contact with the metal, andmolten metal is drawn or transported toward the remaining unreacted massby a wicking or capillary action. This transported metal formsadditional reaction product upon contact with the remaining unreactedmass, and the formation or development of a ceramic body is continueduntil the parent metal or remaining unreacted mass has been consumed, oruntil the reaction temperature is altered to be outside the reactiontemperature envelope. The resulting structure comprises, depending uponthe particular materials comprising the bed or mass which is to bereactively infiltrated, one or more of a parent metal boride, a parentmetal boro compound, a parent metal carbide, a parent metal nitride, ametal (which as used herein is intended to include alloys andintermetallics), or voids, or a combination thereof, and these severalphases may or may not be interconnected in one or more dimensions. Thefinal volume fractions of the reaction products and metallic phases, andthe degree of interconnectivity, can be controlled by changing one ormore conditions, such as the initial density of the mass to bereactively infiltrated, the relative amounts and chemical composition ofthe materials contained within the mass which is to be reactivelyinfiltrated, the amount of parent metal provided for reaction, thecomposition of the parent metal, the presence and amount of one or morefiller materials, temperature, time, etc.

Typically, the mass to be reactively infiltrated should be at leastsomewhat porous so as to allow for wicking the parent metal through thereaction product. Wicking occurs apparently either because any volumechange on reaction does not fully close off pores through which parentmetal can continue to wick, or because the reaction product remainspermeable to the molten metal due to such factors as surface energyconsiderations which render at least some of its grain boundariespermeable to the parent metal.

In another aspect of the first step of the invention, a composite isproduced by the transport of molten parent metal into the bed or masswhich is to be reactively infiltrated which has admixed therewith one ormore inert filler materials. In this embodiment, one or more suitablefiller materials are mixed with the bed or mass to be reactivelyinfiltrated. The resulting self-supporting ceramic-metal composite thatis produced typically comprises a dense microstructure which comprises afiller embedded by a matrix comprising at least one parent metalreaction product, and also may include a substantial quantity of metal.Typically, only a small amount of material (e.g., a small mount of boroncarbide) is required to promote the reactive infiltration process. Thus,the resulting matrix can vary in content from one composed primarily ofmetallic constituents thereby exhibiting certain propertiescharacteristic of the parent metal; to cases where a high concentrationof reaction product is formed, which dominates the properties of thematrix. The filler may serve to enhance the properties of the composite,lower the raw materials cost of the composite, or moderate the kineticsof the reaction product formation reactions and the associated rate ofheat evolution. The precise starting amounts and composition ofmaterials utilized in the reaction infiltration process can be selectedso as to result in a desirable body which is compatible with the secondstep of the invention.

In another aspect of the first step of the present invention, thematerial to be reactively infiltrated is shaped into a preformcorresponding to the geometry of the desired final composite. Subsequentreactive infiltration of the preform by the molten parent metal resultsin a composite having the net shape or near net shape of the preform,thereby minimizing expensive final machining and finishing operations.Moreover, to assist in reducing the amount of final machining andfinishing operations, a barrier material can at least partially, orsubstantially completely, surround the preform. The use of a graphitematerial (e.g., a graphite mold, a graphite tape product, a graphitecoating, etc.) is particularly useful as a barrier for such parentmetals as zirconium, titanium, or hafnium, when used in combination withpreforms made of, for example, boron carbide, boron nitride, boron andcarbon. Still further, by placing an appropriate number of through-holeshaving a particular size and shape in the aforementioned graphite mold,the amount of porosity which typically occurs within a composite bodymanufactured according to the first step of the present invention, canbe reduced. Typically, a plurality of holes is placed in a bottomportion of the mold, or that portion of the mold toward which reactiveinfiltration occurs. The holes function as a venting means which permitthe removal of, for example, argon gas which has been trapped in thepreform as the parent metal reactive infiltration front infiltrates thepreform.

Still further, the procedures discussed above herein in the Section"Discussion of Related Patents and Patent Applications" may beapplicable in connection with the first step of the present invention.

Once a self-supporting body has been formed in accordance with the firststep of the present invention, then the second step of the presentinvention is put into effect. The second step of the present inventioninvolves contacting at least a portion of the formed self-supportingbody with a material which causes at least a portion of the metallicconstituent to be at least partially removed from the self-supportingbody.

In a first embodiment of the invention, a metallic constituent of aself-supporting composite body produced in accordance with the firststep of the present invention can be at least partially, orsubstantially completely, removed by causing the metallic constituent toreact with an adjacent permeable mass of material. To achieve removal ofthe metallic constituent, at least a portion of the permeable mass isplaced into contact with at least a portion of the metallic constituentcontained within the self-supporting body. Thus, at least a portion ofthe metallic constituent should be at least partially accessible, orshould be made to be at least partially accessible, from at least onesurface of the self-supporting composite body.

The amount or selected portion of metallic constituent which is causedto be removed from the self-supporting body can be controlled to achievea desirable metal content. Specifically, substantially all metallicconstituent located in a certain area within a self-supporting compositebody (e.g., located near a surface of the self-supporting compositebody) may be substantially completely removed from that selected area,thereby leaving other areas of metallic constituent within the compositebody substantially undisturbed. Moreover, if the metallic constituent issubstantially interconnected throughout the composite body,substantially all the metallic constituent could be removed. Thevolumetric amount of metallic constituent to be removed from theself-supporting composite body depends upon the ultimate application forthe composite body. Thus, the present invention may be utilized merelyas a surface modification process, or it could be used to removesubstantially all the metallic constituent from a self-supportingcomposite body.

In a preferred embodiment, the second step of the present invention, theself-supporting body may be substantially completely surrounded by andcontacted with an appropriate material. In this embodiment, at least aportion of, or substantially all of, the metallic constituent could beremoved from substantially all surfaces of the self-supporting body solong as the metallic constituent is at least partially accessible, orcan be made to be at least partially accessible from such surfaces.

In another preferred embodiment of the second step of the presentinvention, only a portion of the self-supporting body may be contactedwith the appropriate mass of material. In this preferred embodiment, themetallic constituent could be selectively removed from that surfacewhich is in contact with the permeable mass. In this preferredembodiment, it is possible to achieve a grading of properties within aself-supporting body from one side of the body relative to another sideof the body. Such grading could permit the self-supporting body to beused for a number of different applications.

A number of materials may be placed into contact with self-supportingbodies formed in accordance with the first step of the presentinvention. Acceptable materials include carbide, nitrides, borides, etc.A primary selection criteria for the material comprising the permeablemass is that the permeable mass should be wetable by the metalliccomponent of the self-supporting body. Moreover, the permeable mass canbe selected so that it is substantially nonreactive with or veryreactive with the metallic component of the self-supporting body. In thecase where the permeable mass is selected so that it is substantiallynonreactive with the metallic constituent comprising a self-supportingbody, very little conversion of metallic constituent to another phasecan be expected; whereas if a metallic constituent is reactive with amaterial in the permeable mass, partial conversion of the metallicconstituent to another material can be expected.

As stated above, the amount of metallic constituent that is removed froma self-supporting body can be controlled to be within any particulardesirable range. For example, if a self-supporting body was formed tocontain about 20 volume percent metallic constituent, substantially allof the metallic constituent could be removed by following the teachingsof the present invention. Additionally, it has been observed, that whenthe material comprising the permeable mass is substantially nonreactive(e.g., chemically) with metallic constituent contained in theself-supporting body, that substantially no conversion of metallicconstituent to another material occurs. But rather, substantiallycomplete removal of the metallic constituent from the self-supportingbody is essentially all that occurs. This fact has been proven byquantitative image analysis.

The following are examples of the present invention. The Examples areintended to be illustrative of various aspects of the present invention,however, these examples should not be construed as limiting the scope ofthe invention.

EXAMPLE 1

This Example demonstrates a technique for removing the residual metallicconstituent from a platelet reinforced composite body. A lay-up used toform the platelet reinforced composite body is shown in FIG. 1. Thelay-up used to remove the residual metallic constituent from the formedplatelet reinforced composite body is shown in FIG. 2.

About 600 grams of methylene chloride (JT Baker, Inc., Phillipsburg,N.J.) was poured into an approximately 1/2 gallon (2 liter) NALGENE® jar(Nalge Company, Rochester, N.Y.). About 4 grams of XUS 40303.00Experimental Binder (Dow Chemical Company, Midland, Mich.) was added tothe methylene chloride and allowed to dissolve. About 400 grams of 1000grit TETRABOR® boron carbide particulate (ESK Engineered Materials, NewCanaan, Conn.) having an average particle size of about 5 microns wasstirred into the solution of binder and methylene chloride to form aslurry.

As shown in FIG. 1, a grade ATJ graphite mold 10 (Union CarbideCorporation, Carbon Products Division, Cleveland, Ohio) having innerdimensions measuring about 3.0 inches (76 mm) square and about 4.0inches (102 mm) high, was filled with methylene chloride (JT Baker Inc.,Phillipsburg, N.J.) and placed into a drying box at substantially roomtemperature for about one hour to allow the methylene chloride solventto saturate the graphite mold 10. After soaking for about an hour, theresidual methylene chloride was poured out, and a quantity of the slurrycontaining the boron carbide particulate was sediment cast into thesaturated graphite mold 10. The graphite mold 10 containing the sedimentcast boron carbide preform 12 were placed back into the drying box andallowed to dry overnight.

The graphite mold 10 and the dried sediment cast preform 12 were thenfired in a resistance heated controlled atmosphere furnace to remove thebinder from the preform. Specifically, the graphite mold 10 and itscontents was placed into the furnace chamber, which was then evacuatedto about 30 inches (762 mm) of mercury vacuum, and then back-filled withargon gas. After repeating this evacuation and back-filling procedure,an argon gas flow rate of about two liters per minute at an overpressure of about 1 psi (7 kPa) was established. The furnace temperaturewas then increased from substantially room temperature to a temperatureof about 250° C. at a rate of about 44° C. per hour. Upon reaching atemperature of about 250° C., the temperature was then increased toabout 300° C. at a rate of about 50° C. per hour. Upon reaching atemperature of about 300° C., the temperature was then increased toabout 400° C. at a reduced rate of about 10° C. per hour. Upon reachinga temperature of about 400° C., the temperature was then increased toabout 600° C. at a rate of about 50° C. per hour. After maintaining atemperature of about 600° C. for about four hours, the ceramic binderhad been substantially completely removed from the sediment castpreform, and the furnace was then cooled to substantially roomtemperature at a rate of about 200° C. per hour. After cooling tosubstantially room temperature, the graphite mold 10 and sediment castpreform 12 were removed from the furnace. The weight of the preform 12itself was found to be about 80 grams and the preform thickness wascalculated to about 0.46 inch (12 mm). The bulk density of the sedimentcast boron carbide preform 12 was calculated to about 1.18 grams percubic centimeter, corresponding to a theoretical density of about 46.8%.

About 534.6 grams of zirconium sponge 14, (Consolidated Astronautics,Inc., a division of United-Guardian Corp., Hauppage, N.Y.) was poured ontop of the sediment cast boron carbide preform 12 in the graphite mold10 and the zirconium levelled to form a lay-up.

The lay-up comprising the graphite mold 10 and its contents were placedinto a vacuum furnace. The furnace chamber was evacuated to about 30inches (762 mm) of mercury vacuum and then back-filled with argon gas.After repeating this evacuation and back-filling procedure, an argon gasflow rate of about two liters per minute was established through thefurnace at an over pressure of about 2 psi (14 kPa). The furnacetemperature was then increased from about room temperature to atemperature of about 1900° C. at a rate of about 375° C. per hour. Aftermaintaining a temperature of about 1900° C. for about two hours,reactive infiltration of the boron carbide preform by the moltenzirconium metal was substantially complete. Accordingly, the furnacetemperature was decreased to substantially room temperature at a rate ofabout 900° C. per hour. After cooling to substantially room temperature,the graphite mold 10 and its contents were removed from the furnace. Theplatelet reinforced composite body formed by the reactive infiltrationof the zirconium metal into the sediment cast boron carbide preform 12weighed about 601 grams and measured about 3.0 inches (76 mm) square byabout 0.73 inch (18.5 mm) thick. The formed body comprised zirconiumdiboride, zirconium carbide, and a metallic constituent comprisingresidual zirconium alloy.

A portion of the formed platelet reinforced composite was sectionedusing electro-discharge machining, mounted in a thermoplastic polymerand polished using diamond polishing compound in preparation forexamination by optical microscopy. Quantitative image analysis of thepolished sample showed a residual metal content of about 16.3%.

A coupon 20 of the platelet reinforced composite body was machined fromthe formed 3.0 inches (76 mm) square tile using electro-dischargemachining. The machined coupon 20 weighed about 2.13 grams and measuredabout 21.6 mm long by about 16.4 mm wide by about 1.1 mm thick.

Zirconium carbide particulate 22 (-325 mesh, Atlantic EquipmentEngineers, Bergenfield, N.J.) having substantially all particles smallerthan about 45 microns in diameter was poured into a grade ATJ graphitecrucible 24 (Union Carbide Corporation, Carbon Products Division,Cleveland, Ohio) measuring in its interior about 2.0 inches (51 mm)square by about 3.0 inches (76 mm) high to a depth of about 1.0 inch (25mm). The machined coupon 20 of the platelet reinforced compositematerial was placed flat onto the levelled surface of the zirconiumcarbide particulate material 22. Additional zirconium carbideparticulate 22 was added to the graphite crucible to substantiallycompletely cover the coupon 20 of platelet reinforced composite materialuntil a total depth of zirconium particulate 22 of about 2.0 inches (51mm) was realized.

The graphite crucible 24 and its contents was then placed into aresistance heated controlled atmosphere furnace. The furnace chamber wasevacuated to about 30 inches (762 mm) of mercury vacuum and thenback-filled with argon gas. An argon gas flow rate of about two litersper minute was established through the furnace at an over pressure ofabout 2 psi (14 kPa). The furnace temperature was then increased fromsubstantially room temperature to a temperature of about 1800° C. at arate of about 400° C. per hour. After maintaining a temperature of about1800° C. for about one hour, the temperature was then decreased at arate of about 350° C. per hour. After the temperature had been reducedto substantially room temperature, the graphite crucible 24 and itscontents were removed from the furnace and disassembled.

The coupon 20 of the platelet reinforced composite body which wasrecovered was mounted and polished for examination in the opticalmicroscope in substantially the same manner as was the originally formedplatelet reinforced composite body. FIG. 3 is an approximately 100×magnification photomicrograph showing a surface layer attached to theoriginal platelet reinforced composite body comprising zirconium carbideand some zirconium diboride. Adjacent to the surface layer, is a thinlayer comprising zirconium diboride, zirconium carbide, and porosity butno residual metal. The porosity appeared to be interconnected. Beneaththis thin layer was a microstructure comprising zirconium diboride andzirconium carbide. Quantitative image analysis of the underlyingmicrostructure reported only about 0.26% residual metal and about 0.35%porosity.

The volume fraction ratio of zirconium diboride to zirconium carbidewithin the underyling microstructure as determined also by quantitativeimage analysis, was recorded before and after the second heating toremove the metallic constituent from the body. The ratio was found to besubstantially unchanged, indicating that the residual metal in theplatelet reinforced composite had not carburized upon heating in azirconium carbide environment. Moreover, the almost completedisappearance of residual metal from the original platelet reinforcedcomposite body may be attributed to physical removal of such metal fromthe body. That substantially no porosity was seen in the plateletreinforced composite body following the metal removal process indicatesthat some shrinkage or sintering of the body may have occurred.Specifically, it appeared that the body decreased in volume by about16%.

This Example thus illustrates a technique for removing the residualmetal from a platelet reinforced composite body. The relative amounts ofthe zirconium diboride and zirconium carbide phases are leftsubstantially unchanged. Furthermore, the body may maintainsubstantially full density through a shrinkage mechanism.

What is claimed is:
 1. A method for removing at least a portion of ametallic constituent from a self-supporting composite body, saidself-supporting composite body being made by a process comprising: (i)heating a parent metal in a substantially inert atmosphere to atemperature above its melting point to form a body of molten parentmetal; (ii) contacting said body of molten parent metal with a firstpermeable mass which is to be reactively infiltrated; (iii) maintainingsaid temperature for a time sufficient to permit infiltration of moltenparent metal into said first permeable mass which is to be reactivelyinfiltrated and to permit substantial reaction of said molten patentmetal with said first permeable mass to form at least oneboron-containing compound; (iv) containing said infiltration reactionfor a time sufficient to produce said at least one self-supportingcomposite body comprising at least one parent metal boron-containingcompound and at least one metallic constituent of said parent metalwhich is at least partially accessible from at least one surface of saidcomposite body, said method comprising the steps of:contacting at leasta portion of said at least one self-supporting composite body with asecond permeable mass which is capable of reacting with at least aportion of said metallic constituent in said self-supporting compositebody; heating said at least one self-supporting composite body and saidsecond permeable mass to cause at least a portion of said at least onemetallic constituent to infiltrate and react with at least a portion ofsaid second permeable mass to form a reaction product of said at leastone metallic constituent exterior to said composite body, therebyremoving said at least a portion of said at least one metallicconstituent from said self-supporting composite body; and continuingsaid removing of said at least one metallic constituent for a timesufficient to remove a desired amount of said at least one metallicconstituent.
 2. The method of claim 1, wherein said permeable masssubstantially completely surrounds said composite body.
 3. The method ofclaim 1, wherein said permeable mass contacts substantially only oneside of said composite body.
 4. The method of claim 1, wherein saidpermeable mass comprises a ceramic particulate.
 5. The method of claim4, wherein the metallic constituent of the composite body is selectivelyremoved from only that portion of the composite body which contacts saidpermeable mass.
 6. The method of claim 1, wherein substantially all ofsaid metallic constituent is removed.
 7. The method of claim 1, whereinsaid metallic constituent is selectively removed from only a portion ofthe composite body such that a grading of at least one property withinsaid self-supporting body is achieved.
 8. The method of claim 1, whereinsaid second permeable mass comprises at least one material selected fromthe group consisting of carbides, borides, and nitrides.
 9. The methodof claim 1, wherein said parent metal comprises at least one metalselected from the group consisting of hafnium, titanium and zirconium.10. The method of claim 1, wherein said first permeable mass comprisesat least one material selected from the group consisting of boroncarbide, boron nitride, and mixtures of boron-donor materials andcarbon-donor materials.
 11. The method of claim 1, wherein said parentmetal comprises at least one metal selected from the group consisting ofaluminum, silicon, titanium, hafnium, lanthanum, iron, calcium,vanadium, niobium, magnesium and beryllium.
 12. The method of claim 1,wherein said first permeable mass further comprises at least one inertfiller material.
 13. A method for removing at least one metalliccomponent of a metallic constituent contained within a multi-phasecomposite body comprising:contacting at least a portion of a surface ofsaid multi-phase composite body with a permeable mass, said at least onemetallic constituent being capable of reacting with at least a portionof said permeable mass; and infiltrating and reacting at least a portionof the permeable mass with said at least one metallic component of saidmetallic constituent, thereby reducing the amount of metallicconstituent in the multi-phase composite body.
 14. The method of claim13, wherein said permeable mass substantially completely surrounds saidcomposite body.
 15. The method of claim 13, wherein said permeable masscontacts substantially only one side of said composite body.
 16. Themethod of claim 13, wherein said permeable mass comprises a ceramicparticulate.
 17. The method of claim 16, wherein the metallicconstituent of the composite body is selectively removed from only thatportion of the composite body which contacts said permeable mass. 18.The method of claim 13, wherein substantially all of said metallicconstituent is removed.
 19. The method of claim 13, wherein saidmetallic constituent is selectively removed from only a portion of thecomposite body such that a grading of at least one property within saidself-supporting body is achieved.
 20. The method of claim 13, whereinsaid permeable mass comprises at least one material selected from thegroup consisting of carbides, borides, and nitrides.